1. Field of the Invention
This invention relates generally to medical devices, and, more particularly, to methods, apparatus, and systems for limiting patient-initiated electrical signal therapy by a medical device.
2. Description of the Related Art
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. Nos. 4,702,254, 4,867,164, and 5,025,807 to Dr. Jacob Zabara, which are hereby incorporated in this specification in their entirety by reference.
More generally, the endogenous electrical activity (i.e., activity attributable to the natural functioning of the patient's own body) of a neural structure of a patient may be modulated in a variety of ways. In particular, the electrical activity may be modulated by exogenously applied (i.e., from a source other than the patient's own body) electrical, chemical, or mechanical signals applied to the neural structure. The modulation (hereinafter referred to generally as “neurostimulation” or “neuromodulation”) may involve the induction of afferent action potentials, efferent action potentials, or both, in the neural structure, and may also involve blocking or interrupting the transmission of endogenous electrical activity traveling along the nerve. Electrical signal therapy or electrical modulation of a neural structure (also known as “electrical signal therapy”) refers to the application of an exogenous therapeutic electrical signal (as opposed to a chemical or mechanical signal), to the neural structure. Electrical signal therapy may be provided by implanting an electrical device underneath the skin of a patient and delivering an electrical signal to a nerve such as a cranial nerve. The electrical signal therapy may involve performing a detection step, with the electrical signal being delivered in response to a detected body parameter. This type of stimulation is generally referred to as “active,” “feedback,” or “triggered” stimulation. Alternatively, the system may operate without a detection system once the patient has been diagnosed with epilepsy (or another medical condition), and may periodically apply a series of electrical pulses to the nerve (e.g., a cranial nerve such as a vagus nerve) intermittently throughout the day, or over another predetermined time interval. This type of stimulation is generally referred to as “passive,” “non-feedback,” or “prophylactic,” stimulation. The stimulation may be applied by an implantable medical device that is implanted within the patient's body.
Generally, implantable medical devices (IMD) are capable of receiving a signal that may affect the operation of the IMD from an external source, such as a patient-initiated signal or a signal in the patient's environment. For example, a patient-initiated signal may be used to activate a predetermined function of the IMD, such as providing additional therapeutic electrical signals to a cranial nerve of the patient. A magnetic sensor may be provided in the IMD to detect a significant magnetic field, either from the patient or the patient's environment (for example an MRI machine) and in response, the IMD may activate a predetermined function. A patient-initiated signal such as a magnetic field may be an inhibitory input or an excitatory input. The inhibitory input may relate to inhibiting a function normally performed by the IMD. For example, application of a first patient-initiated magnetic field to the IMD may cause delivery of the electrical signal from the IMD to the nerve to be inhibited for a certain time period. Conversely, application of a second patient-initiated magnetic field signal to the IMD may comprise an excitatory input that may prompt the IMD to perform additional functions. For example, additional electrical signal therapy may be performed by the IMD based upon a second patient-initiated magnetic signal input. The magnetic signal input may be generated by a patient by placing a magnet proximate the skin area under which the implantable medical device resides in the human body. Both types of magnetic field signals are typically referred to as “magnet modes” or as “magnet mode” operation. Other types of patient-initiated signals, such as a tap signal on the IMD, or a radio-frequency (RF) signal provided to the device, may be used in addition to magnetic field signals.
One problem associated with the state of the art is that optimal safety and efficacy of neurostimulation by an IMD may be compromised by allowing a patient to initiate neurostimulation ad libitum. For example, excessive delivery of electrical current to a neurological structure may result in a net electrical charge on, and subsequent damage to, the target neurological structure. Current doses that do not lead to tissue damage may lead to discomfort. Also, extra dosing may lead to reduced efficacy by interfering with short-term or long-term recovery periods necessary or advisable for proper and/or safe neural functioning. Further, battery life can be shortened by excessive dosing. On the other hand, blanket prohibition of patient-initiated neurostimulation may lead to ineffective alleviation of symptoms during acute crisis, when extra dosing may be helpful, e.g., during an epileptic seizure suffered by a patient having an implanted vagus nerve stimulation (VNS) device. Also, blanket prohibition of patient-initiated neurostimulation, if communicated to the patient, may remove any placebo effect benefit the patient may receive from signaling the IMD to perform neurostimulation via the actions of passing the magnet over the IMD or tapping the IMD, among other modes of initiating the therapeutic electrical signal.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.